1. Field of the Invention
The present invention relates to a control system for an engine of an automotive vehicle equipped with a lock-up type automatic transmission, and more particularly to a control system for exhaust gas recirculation and ignition timing of the engine.
2. Description of the Prior Art
Most automatic transmissions include a torque converter in their power transmission system thereof so as to increase the torque fed to the transmission from an engine. Most torque converters are of the type wherein a pump impeller driven by the engine causes the rotation of operating oil within the torque converter so as to rotate a turbine runner under the reaction of a stator to thereby increase the torque (torque-converter state). During the operation of the torque converter, a certain amount of slip between the pump impeller and turbine runner is unavoidable. Thus the automatic transmission which include such a torque converter in that power transmission system have an advantage in that they are easy to operate, but suffer a drawback in that the power transmission effeciency is poor, resulting in poor fuel economy. To alleviate this drawback, torque converters have been proposed torque having a direct clutch (also called a lock-up torque converter) wherein during relatively high ranges of vehicle speed operation where torque variation of an associated engine do not create a problem, the turbine runner is directly connected to the pump impeller (lock-up state) to eliminate any slip therebetween. Recently some vehicles have used a lock-up type automatic transmission including a torque converter of this kind in its power transmission system.
Since a vehicle equipped with an ordinary automatic transmission has a torque converter in its power transmission system that is subjected to a slip, as described before, relatively high engine speeds are required to obtain the same running state as compared to the vehicle equipped with a manual transmission having no such slip. Therefore, it is a common practice to increase the exhaust gas recirculation and to advance the ignition timing of engines having such automatic transmissions in comparison to vehicles equipped with a manual transmission, for the purpose of exhaust gas purification. The exhaust gas recirculation and the ignition timing settings of lock-up type automatic transmissions are similarly increased and advanced respectively.
However, if the exhaust gas recirculation setting is so increased and the ignition timing setting are so advanced, it may cause unstable combustion when the automatic transmission operates in the lock-up state, where power transmission operation is similar to the manual transmission operation, thus causing surging during crusing operation. Moreover, where the ignition timing is advanced too much under such operating conditions, it may cause knocking. The result is a failure to satisfactorily accomplish the intended object of lock-up type of the automatic transmissions, viz., enhancement of fuel economy.